1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which performs a screen process on image data of a plurality of colors using a threshold matrix for each color.
2. Description of Related Art
As a method for reproducing a medium tone of an image, frequency modulation (FM) screening and amplitude modulation (AM) screening have conventionally been known. FM screening expresses a shade by the density of dots, each dot having the same size. On the other hand, AM screening forms halftone dots, each consisting of a group of dots, at regular intervals. AM screening expresses a shade by varying the sizes of the halftone dots.
A screen pattern of AM screening has periodicity of halftone dots. For this reason, when screen patterns of a plurality of colors are superimposed with one another to reproduce a color image, moiré tends to arise owing to interference. Therefore, it is common to vary a screen angle depending on the color in order to reduce moiré.
As another method to reduce moiré, Japanese Patent Application Laid-Open No. 2004-135317 proposes to use FM screening for at least one of a plurality of colors to reduce the number of colors for which AM screening is used.
A screen pattern formed with FM screening has high resolution and the dots thereof are positioned at random, in general, which tends to cause dot gain and leads to low stability of density.
Such a combined use of FM screening and AM screening for reproducing color images causes a decrease in the stability of density of a color for which FM screening is used, among the colors. This may lead to reduction in image quality of the entire color image. Therefore, there has been a demand for a more effective way of reducing moiré.